Two- and Three-Dimensional Fabrication Techniques
Conventional photolithography is believed to be limited to about 150 nm in pattern dimensions. While X-ray and ion beam lithography have been demonstrated as viable alternative techniques for creating pattern dimensions below this limit, they are expensive. E-beam lithography has also been proven as a viable technique. However, it is time consuming and, like X-ray and ion beam lithography, expensive. In contrast to such lithographic techniques, imprinting offers an attractive alternative to the fabrication of two-dimensional (2-D) nanometer-scale features, as a result of simpler, faster, and cheaper processing, making this technique a potential replacement for photolithography in mass production.
The above-mentioned lithographic techniques are further limited to fabrication of 2-D and supported features. Imprinting, however, can lend itself to the fabrication of three-dimensional (3-D) features, wherein 3-D features comprise structural variation with depth. Three-dimensional patterning techniques are likely to be important enabling technologies for a number of applications. In microelectronics, for example, the third dimension could possibly allow the speed and memory of microprocessors to go beyond the limitations currently imposed by 2-D features. In optoelectronic industries, 3-D photonic band gap structures are garnering considerable attention because 3-D structures serve to minimize loss of light (Kiriakidis et al., “Fabrication of 2-D and 3-D Photonic Band-Gap Crystal in the GHz and THz Region,” Mater. Phys. Mech., 1:20-26, 2000). In drug/chemical delivery systems, sensing systems and catalysis, the feasibility of fabricating 3-D structures will provide breakthroughs in the efficiency of controlled delivery, sensing, and selectivity in chemical reactions. For example, a sphere with a meshed surface can be envisioned as a chambered pill that contains multiple drugs or a multifunctional catalysis support.
While 2-D fabrication techniques are mature technology down to the sub-micrometer scale, very little has been reported regarding 3-D sub-micrometer fabrication techniques. Currently, of the limited amount of literature available on sub-micrometer 3-D fabrication techniques, most reports are seen to be mere extensions of various photolithography techniques. For instance, Whitesides et al. have shown that a porous microsphere can be obtained via a self-assembly approach (Huck et al., “Three-Dimensional Mesoscale Self-Assembly,” J. Am. Chem. Soc., 129:8267-8268, 1998), and Yamamoto et al. have demonstrated the fabrication of micrometer scale grooved structures using deep X-ray lithography (Tabata et al., “3D Fabrication by Moving Mask Deep X-ray Lithography with Multiple Stages,” The Fifteenth IEEE International Conference on Micro Electro Mechanical Systems, 180-183, 2002). Whitesides et al. have also reported on a “membrane folding” method used to create 3-D structures (Brittain et al., “Microorigami: Fabrication of Small Three-Dimensional Metallic Structures,” J. Phys. Chem. B, 105:347-350, 2001). While most of these techniques have demonstrated the feasibility of creating 3-D sub-micrometer or nanometer scale features, they are not easily implemented for mass production.
Both conventional nano-imprinting (Sun et al., “Multilayer resist methods for nanoimprint lithography on nonflat surfaces,” J. Vac. Sci. Technol. B, 16(6):3922-3925, 1998) and reversal imprinting (Huang et al., “Reversal imprinting by transferring polymer from mold to substrate,” J. Vac. Sci. Technol. B, 20(6):2872-2876, 2002) techniques are attractive alternatives to the above-mentioned techniques in the fabrication of 3-D nano-features, although currently both techniques create 3-D structures through multiple imprinting on patterned substrates or on substrates with topology. A more efficient imprinting technique would, therefore, go a long way in solidifying imprinting's role as a potential replacement for currently used lithographic patterning techniques.
Organic Light-Emitting Devices
Organic light emitting diode (OLED)-based devices, which make use of thin film materials that emit light when excited by electric current, are becoming an increasingly popular technology for applications such as flat panel displays. Pixilated OLED devices can be used as displays in various consumer electronic products including cellular phones, personal organizers, pagers, advertising panels, touch screen displays, and other teleconferencing and multimedia products.
Referring to FIG. 11, an OLED device typically comprises a functional stack of one or more organic (e.g., polymer) functional layers 1101 between a patterned transparent substrate (e.g., indium-tin-oxide “ITO” glass) 1102, which acts as a transparent anode, and a metallic top electrode 1103. Typically, the functional stack is formed on a transparent substrate 1102, and conductive layers 1104 are patterned to form parallel rows of cathodes 1104(a) in one direction and columns of anodes 1104(b) in a direction perpendicular to the cathodes. OLED pixels are located where the cathodes and anodes overlap. During operation, charge carriers are injected through the cathodes and anodes for recombination in the functional layers. The recombination of the charge carriers causes the functional layer of the pixels to emit visible radiation.
To define pixels and other patterns on a display, the ITO, organic layers, and cathode must be patterned. Patterning the ITO anode layer is relatively easy and may be carried out by conventional photolithography. Such patterned ITO anode layers are generally provided to OLED manufacturers by their suppliers pre-patterned to their specifications without much additional cost. To make a display with high resolution and high filling factor, however, the spacing between pixels must be defined by the patterning processes that formed the organic layers and the cathodes, and these patterning processes typically must be sub-100 μm in feature resolution. Various conventional patterning techniques have been used to form the cathodes, such as shadow masking, photolithography (with wet or dry etching), laser ablation, and lift-off techniques (wet or dry resists).
OLED-based displays fabricated using shadow-mask technology (FIG. 11) are difficult to fabricate because one must accurately align multiple layers of deposited material using shadow masks, and the masks tend to clog. Moreover, it is difficult to fabricate features smaller than about 300 μm by 300 μm using a shadow mask, whereas OLEDs smaller than about 100 μm by 100 μm, and possibly smaller than about 10 μm by 10 μm, are generally desirable for high-resolution, full-color flat panel displays.
Direct photolithographic patterning is generally used as the standard method for patterning materials and fabricating devices on a submicron scale, wherein features much smaller than those achievable with shadow-mask technology can be fabricated. However, the organic materials used to fabricate OLEDs may be quickly degraded from exposure to deleterious substances such as water, solvents, developers, and even atmospheric conditions. In particular, many of the chemicals used in photolithographic processing, such as solvents and developers used to wash away photoresist, may rapidly degrade such organic materials. To circumvent this drawback, as shown in FIG. 12, manufacturers are currently employing a “self-patterning” approach in which two layers of photoresist are patterned on ITO using microlithography to create a topology that interrupts the continuity of the organic layer and the cathode films into discrete rows (or columns) when the cathode is deposited. Referring to FIG. 12, the T-bar column structure 1201 is one type of structure that is capable of correctly shadowing the organic LED materials 1202 and the cathode materials 1203 in this fashion. Other types of structures that have been demonstrated include reverse tapered columns or inverted pyramidal structures. While this methodology provides high-resolution patterns, it requires OLED manufacturers to invest in expensive photolithographic equipment and manpower just to make the substrates for their displays.
While M. Li et al. have shown that it is possible to fabricate T-bar column structures using conventional nanoimprint lithography (Li et al., “Direct three-dimensional patterning using nanoimprint lithography,” App. Phys. Lett., 78(21):3322-3324, 2001), the entire structure must be fabricated in a single mold, which is not easy to achieve and requires a complex fabrication scheme along with e-beam lithography, and is thus not suitable for imprinting over large areas. The final structure is also fabricated by metal vapor deposition and lift-off of a sacrificial layer to leave behind metallized T-bar columns. Metallic T-bar structures are not suitable for display manufacturing because they are conductive in nature and are far more likely to short out pixels if the structure does not shadow evaporated materials properly, or if the display has defects or is damaged while in service. Since polymers are insulative in nature, polymer T-bar structures are more desirable for display manufacturing purposes.
Y. Chen and D. Moran et al. have also fabricated metallized T-gates for high-speed transistors (Chen et al., “A non-destructive method for the removal of residual resist in imprinted patterns,” Microelectronic Engineering, 67-68:245-251, 2003; Moran et al., “Novel technologies for the realisation of GaAs pHEMTs with 120 nm self-aligned and nanoimprinted T-gates,” Microelectronic Engineering, 67-68:769-774, 2003). These structures are similar to those described in Li et al. above, but are directed toward a different application. They do, however, suffer from the same drawbacks as the structures of Li et al. Metallized T-bar structures are also unsuitable for flexible displays on polymer structures due to the fact that bending the display will induce irreversible plastic deformation that could cause the metallized columns to short out pixels.
As a result of the foregoing, a technique suitable for fabricating polymeric T-bar structures would be very beneficial for OLED and other related devices.